miri-script: use --remap-path-prefix to print errors relative to the right root

[RalfJung]

Inspired by rust-lang/rust-clippy#13232, this makes it so that when cargo-miri fails to build, ./miri check will print errors with paths like cargo-miri/src/setup.rs. That means we can get rid of the miri-symlink-hacks and instead tell RA to just always invoke the ./miri clippy script just once, in the root.

This means that we can no longer share a target dir between cargo-miri and miri as the RUSTFLAGS are different to crates that are shared in the dependency tree need to be built twice with two different flags. miri-script hence now has to set the MIRI environment variable to tell the cargo miri setup invocation where to find Miri.

I also made it so that errors in miri-script itself are properly shown in RA, for which the ./miri shell wrapper needs to set the right flags.

[RalfJung]

All right, this seems to be working now.

@rust-lang/miri what do you think?
Also note that when this lands, you'll have to update your RA config.

[RalfJung]

Oh, looking at the Miri thing again we could also use a totally different approach... namely, to set this as a per-package variable in the profile, which will also avoid applying it to dependencies.

[saethlin]

Why do we now need this?

[saethlin]

I suppose more than anything I'm confused that we didn't need this before.

[RalfJung]

We now need this because miri-script needs nightly because it picks up the .config/cargo.toml which uses nightly features.

Previously we didn't need this because miri-script was built with stable, and then it used rustup-toolchain-install-master to install the toolchain used for Miri.

[saethlin]

Ah.

[saethlin]

@rust-lang/miri what do you think?

No objections. I trip over the path prefixes more in the rust-lang/rust tree than this repo, but still a nice change.

[RalfJung]

Okay let's land this then. :)
@bors r=saethlin

[bors]

📌 Commit b9ebd52 has been approved by saethlin

It is now in the queue for this repository.

[bors]

⌛ Testing commit b9ebd52 with merge c73f658...

[bors]

☀️ Test successful - checks-actions
Approved by: saethlin
Pushing c73f658 to master...